This invention is related to an improved multi-wavelength etalon. More specifically, this invention is related to an etalon having a varied index of refraction.
In many applications, it is necessary to accurately determine the wavelength(s) of light incident on a suitable detector. A widely used type of detector includes an etalon to filter specific frequencies of light. An etalon is a type of interference filter in which the intensity of transmitted light is dependent on its wavelength.
FIG. 1 is a illustration of a conventional etalon 10. The etalon has two partially reflective parallel surfaces 12, 14 which are separated by a distance d and is comprised of a material 16 with an index of refraction r. When collimated light having a wavelength xcex is passed through the etalon, some of the light is reflected between the surfaces 12, 14. The multiply reflected light beams interfere with each other, either constructively or destructively, and thus alter the overall intensity of the light which passes through the etalon 10, e.g., as measured by photodetector 18.
Maximum transmission occurs when twice the distance between the reflective surfaces 12, 14 is an integral number of wavelengths xcex in the etalon. In other words, 2d*r/xcex=x, where x is an integer. The transmission characteristic of etalon 10 is a periodic function of wavelength and the percentage of reflectivity R of the partially reflective surfaces 12, 14. The thickness d and refractive index r of the etalon determine the distance between the transmission peaks around a given wavelength. The reflectivity R of the surfaces determines the percentage of the light that is reflected by the etalon walls. This defines the amount of light which is available for constructive and destructive interference, and thus how narrow or broad the transmission peaks are.
Often, it is desirable to provide a light sensor which is sensitive to, and can discriminate among, several different frequencies of incident light at the same time, which frequencies may be closely spaced. Such a sensor is useful in applications including spectrographic analysis and precise tuning of lasers for use in, e.g., multi-laser fiber optic communication systems. Although several discrete etalons can be utilized for these purposes, in some implementations, a stepped etalon 20 is used instead. In this arrangement, illustrated in FIG. 2, one or both active (i.e., reflective) surfaces of the etalon are stepped so that each step on the etalon provides a region of different thickness. By adjusting the thicknesses appropriately, each step can be configured to pass different frequencies of light. Stepped spectrographic etalon arrangements of this type are shown in U.S. Pat. No. 4,822,998 to Yokota et al. and U.S. Pat. No. 5,144,498 to Vincent.
In use, the stepped etalon 20, having partially reflective coatings 11a, 11b, is positioned over an appropriately configured array of photodetectors 18a, 18b, where each detector is aligned with the land 13a, 13b of a corresponding etalon step. When a beam of light is directed onto the etalon 20, the intensity of the output signal attributed to each detector 18a, 18b indicates the intensity of light passing through the etalon in the region of the corresponding step. Thus, the detector outputs provide a measure of the intensity of incident light with the particular frequencies passed by the various thicknesses of the etalon at each step.
A disadvantage inherent in the use of stepped etalons is that it can be difficult to form the steps such that each step land is optically flat because the existence of steps makes polishing the separate land surfaces difficult. Another drawback to a stepped etalon configuration is the interference caused by the abrupt transition between the lands of adjacent steps. In a conventional unstepped etalon, the intensity within a collimated light beam transmitted through an etalon has the same intensity pattern as the incident beam, typically gaussian-like. However, when an abrupt step is present, the incident and resonant light, in addition to being reflected, is diffracted. This produces interference within the transmitted beam along an axis approximately perpendicular to the step edge. The result of the diffraction is that in the vicinity of the step, there is substantial angular dispersion of the light which reduces the quality of the transmission function. This is evidenced by reduced signal amplitude and broadened peaks.
The effect of the interference and overall reduction in etalon quality associated with abrupt steps also creates a xe2x80x9cdead spotxe2x80x9d behind and near the step edge in which accurate intensity readings are compromised. Thus, there are portions of the etalon where a detector cannot be placed due to the reduced quality of the transmitted beam. For example, experiments using an etalon with a thickness of approximately 2 mm and a step height of approximately 0.2 um indicate that there is a dead spot approximately 600 to 800 um wide directly behind the step. Since input beam widths of between 0.5 to 5.0 mm are common, a significant portion of the transmitted beam will not have high quality etalon transmission characteristics and thus will not be suitable for detection. Having a dead spot in the beam reduces the available optical power for measurement and lowers the power-per-detector. Since a minimum signal-to-noise ratio is required for reliable measurements, decreasing the power-per-detector thus can decrease the accuracy of the detector and the stability of equipment which is adjusted according to the etalon measurements.
Accordingly, it would be advantageous to provide an etalon which is sensitive to multiple discrete wavelengths and which can easily be polished to optical flatness on both sides. It would be further advantageous if such an etalon had only limited interference between regions sensitive to the various discrete wavelengths.
According to the invention, an etalon is formed having at least two regions with different indices of refraction. The change in refractive index alters the wavelengths of incident light in the etalon. This results in changes in the number of wavelengths between the reflective surfaces of the etalon, and therefore, changes in the etalon""s transmission characteristics. Such an etalon can be configured such that the transmission characteristics for the different regions provide for peaks and troughs at preselected wavelengths. Because multi-wavelength selectivity can be achieved without the use of steps, the reflecting surfaces of the etalon can be easily polished a high optical quality flatness using conventional techniques. Further, such an etalon can be thinner than a multi-wavelength etalon which utilizes steps. Preferably, the transition between two adjacent regions with different indices of refraction is graded to reduce optical interference which may result from an abrupt transition.